Plasmonic Electronic Paper

Micro- and nanostructures are widely used by nature to create vivid colors. An example of this is the morpho butterfly with its blue wings created not by a pigment, but by a nano-structure. Adjacent to this, humans have generated colors by plasmonics for thousands of years. An example of this is the metal nanoparticles in old church windows whose resonances create the colorful glasses which still can be seen today.<br/><br/>In our group, we use plasmonic structural colors to create highly reflective pixels in red, green, and blue. We later modulate the reflection with an electrochromic material incorporated inside or on top of the structures.<br/><br/>The plasmonic metasurfaces are made by an evaporated back reflector made of silver or aluminum (~100 nm) and a spacer layer of aluminum oxide (60-110 nm). A thin film (20 nm) of semi-transparent gold is evaporated on top to create a Fabry-Pérot cavity resonator that extinguishes one part of the visible spectra. Patterns such as a nanohole array can be introduced in the gold film by colloidal lithography. This generates a plasmonic effect that scatters red light to further enhance the coloration.<br/>Both tungsten oxide and conjugated polymers are used to modulate the reflection and shut the pixel ON and OFF. The inorganic tungsten oxide gives the best contrast for all three colors¹. However, the switching speed was faster for the conjugated polymers, under some conditions reaching video speed².<br/><br/>By substituting the aluminum oxide spacer layer with an electrochromic material, the metasurface can be dynamically tuned. Tungsten oxide’s electrochromism is accompanied by a change in the real part of the refractive index which alters the resonant frequency of the cavity³. A similar effect is achieved by the thickness change of conjugated polymers upon doping⁴. Tungsten oxide can alter the colors by shifting the peak reflection wavelength around 100 nm while the polymer alternative (PT34bT) can span the whole visible spectra ^{3, 4}.<br/><br/>To further show how metasurfaces could be incorporated into a reflective display the colored structures were prepared on a commercial flexible TFT backplane. Conjugated polymers can then be selectively polymerized on individual pixels and can be controlled individually without cross-talk.<br/><br/>As an alternative to active matrix addressing, we are also working on a passive matrix that has the potential to greatly simplify the fabrication of pixelated devices. The configuration utilizes an electrochemical reaction on a counter electrode based solely on ITO. The reaction on ITO introduces hysteresis into the electrochromic device which makes it possible to use in a passive matrix configuration. The reaction which occurs is likely associated with trace amounts of water.<br/><br/>1. Gugole, M.; Olsson, O.; Xiong, K.; Blake, J. C.; Montero Amenedo, J.; Bayrak Pehlivan, I.; Niklasson, G. A.; Dahlin, A., High-Contrast Switching of Plasmonic Structural Colors: Inorganic versus Organic Electrochromism. <i>ACS Photonics </i><b>2020</b>.<br/>2. Xiong, K.; Olsson, O.; Svirelis, J.; Palasingh, C.; Baumberg, J.; Dahlin, A., Video Speed Switching of Plasmonic Structural Colors with High Contrast and Superior Lifetime. <i>Advanced Materials </i><b>2021</b>, 2103217.<br/>3. Gugole, M.; Olsson, O.; Rossi, S.; Jonsson, M. P.; Dahlin, A., Electrochromic Inorganic Nanostructures with High Chromaticity and Superior Brightness. <i>Nano Letters </i><b>2021,</b> <i>21</i> (10), 4343–4350.<br/>4. Rossi, S.; Olsson, O.; Chen, S.; Shanker, R.; Banerjee, D.; Dahlin, A.; Jonsson, M. P., Dynamically Tuneable Reflective Structural Coloration with Electroactive Conducting Polymer Nanocavities. <i>Advanced Materials </i><b>2021</b>, 2105004.